As the recent wide spread of personal computers and internet communication, the demand for the transmission of information has rapidly been increased. For this reason, there has been desired for the spread of the optical transmission means whose transmission speed is quite high even to the terminal information processing devices such as personal computers. It is thus necessary that a high quality optical waveguide for the optical interconnection should be produced in a large scale at a low price.
As materials for forming optical waveguides, there have been known inorganic materials such as glass and semiconductor materials and resins. An optical waveguide of an inorganic material is in general produced according to a method, which comprises the steps of forming a film of an inorganic material using a film-forming device such as a vacuum vapor deposition device or a sputtering device and then etching the inorganic film thus formed into an optical waveguide having a desired shape. However, the vacuum vapor deposition device or the sputtering device requires the use of an evacuation device and therefore, a large-sized and expensive apparatus should be used. Moreover, this method requires the use of an evacuation step and this makes the process more complicated. On the other hand, when preparing an optical waveguide using a resin, the film-forming process or coating and heating steps can be conducted under the atmospheric pressure and therefore, the method, which makes use of a resin, is advantageous in that quite simple device and process can be used.
Although, there have been known various kinds of resins, which can be used for forming the core and clad layers of an optical waveguide, those particularly expected are polyimides each having a high glass transition point (Tg) and excellent in the heat resistance among others. When forming the core and clad layers of an optical waveguide from a polyimide, the resulting optical waveguide would be expected as one having long-term reliability and an ability of withstanding soldering.
Such an optical waveguide of a polymer material can, for instance, be produced by a method comprising the steps of forming a lower clad layer on a substrate of, for instance, silicon; forming a first film of a resin on the lower clad layer; patterning the first resin film into a shape of a desired optical waveguide to thus form a core layer; coating the surfaces of the lower clad layer and the core layer with a solution containing a material for forming a second resin film according to the spin-coating method; and then drying the coated layer to thus form an upper clad layer consisting of the second resin film.
Thus, the use of resins as materials for forming clad and core layers would permit the production of an optical waveguide made of a resin by a simple production process, but air bubbles are sometimes formed at the boundary between the core layer and the upper clad layer and/or within the upper clad layer, in the area wherein at least two optical waveguides are arranged at quite narrow spaces encountered in, for instance, the branched portion of an optical waveguide having a Y-shaped branch. The generation of such air bubbles would adversely affect the transmission efficiency of the resulting optical waveguide.